Injection-molded articles of thermoplastic resins are used in a wide variety of fields because of their high freedom of shape and excellent mass productivity. With the advancement of thermoplastic molding materials in recent years, various functions have been imparted to injection-molded articles, and so such molded articles have come to be used even as, for example, parts substitutive for the conventional metallic materials. However, the resin materials have an extremely low heat conductivity compared with the metallic materials, so that their developments in, for example, application fields of which good heat radiating property is required are limited.
More specifically, for example, a poly(phenylene sulfide) resin (hereinafter abbreviated as "PPS resin") is an engineering plastic excellent in heat resistance, flame retardancy, chemical resistance, dimensional stability, mechanical properties, electrical properties and the like, and use applications of injection-molded articles thereof are spread as electrical and electronic parts, precision instrument parts, automotive parts, etc. and are also developed into fields in which metallic materials have heretofore been used. However, the PPS resins have an extremely low heat conductivity, and so molded articles thereof are poor in thermal conductibility and heat radiating property and fail to efficiently radiate heat generated from various instruments and apparatus. As a result, in some cases, instruments and apparatus equipped with a part made of such a PPS resin may accumulate heat to a high temperature or have disadvantages such as melting, deterioration, decomposition, deformation, cracking and or the like of the PPS resin-made part.
Accordingly, it is desired to impart high thermal conductibility to heat-resistant thermoplastic resins such as PPS resins so as to be able to apply them to fields of various molded articles, of which good thermal conductibility and heat radiating property are required.
A microplate is an example of a resinous molded article of which high thermal conductibility is required. Various kinds of plastic microplates are used upon the analysis of a biochemical or biological specimen such as DNA, RNA or cells. A number of wells (recessed compartments) is generally provided in such a microplate. A variety of operations such as amplification of DNA, cell growth, and detection and quantitative determination of various materials or specimens is conducted in such wells. However, many of the conventional plastic microplates have not been suitable for use at a high temperature, since they are made from a general-purpose polymer such as polystyrene or polypropylene and are hence insufficient in heat resistance. It has also been difficult to heat such a plastic microplate to a target temperature in a short period of time due to their low heat conductivity.
These problems will hereinafter be described by specific examples. A polymerase chain reaction (PCR) method is a method in which an intended DNA in a specified region is amplified in a great amount from a small amount of a DNA specimen, and is applied to genetic diagnosis techniques, isolation and analysis of unknown genes, etc. In the PCR method, artificial single-stranded DNA fragments of 15 to 20 bases, which will serve as respective complementary primers to both 3'-terminal sides of a double-stranded DNA (100 to several thousands bases), are provided in great amounts. A tube is charged with a DNA intended to amplify, a thermophilic DNA polymerase such as a Tag polymerase and 4 kinds of deoxynucleotide triphosphates, and the mixture is heat treated at 93-96.degree. C. for 30 seconds to 1 minute. By this treatment, the double-stranded DNA is converted into single strand DNAs due to dissociation of hydrogen bond (thermal denaturation of DNA).
When the temperature of this tube is dropped to 45-55.degree. C., the complementary DNAs present in the great amounts each form a hydrogen bond at 3'-terminals of both single-stranded DNAs (annealing of primer). When the temperature is then raised to 70-72.degree. C., a complementary DNA is synthesized from each 3'-terminal by the DNA polymerase using the short complementary DNA as a primer (elongation by DNA polymerase). In such a manner, a molecule of the DNA grows into 2 molecules of the DNA by the synthesis of a new complementary DNA. This cycle (thermal denaturation.fwdarw.annealing.fwdarw.elongation) is conducted repeatedly, whereby molecules of the n-th power of 2 come to be synthesized from 1 molecule after completion of the n-th cycle. One cycle is generally completed in about 2 to 5 minutes. In many cases, the cycle is repeated 20 to 35 times.
In the case of DNA diagnosis or the like, it is adopted to conduct the PCR method with a 96-well microplate. According to this method, the results of hybridization as to many specimens can be determined at once. In addition, the quantitative determination of the results can be conducted with ease.
In the PCR method, the step of holding the microplate at a temperature of about 95.degree. C. for a short period of time is conducted at every cycle. In order to allow the thermal denaturation reaction to uniformly proceed, it is necessary for a specimen to reach the target temperature in a shorter period of time. However, the plastic microplates provided with a number of wells heretofore used in the PCR method have a low heat conductivity in addition to insufficient heat resistance, so that it has been difficult to heat them to a temperature of about 95.degree. C. in a short period of time and to uniformly and efficiently conduct repeated temperature rise and temperature drop.
As methods for heating reaction wells in a plastic microplate used in the PCR method up to the target temperature in a shorter period of time, there have heretofore been used, for example, (1) a method in which the plastic microplate is brought into contact with a heating plate made of an aluminum block, (2) a method in which a copper block is attached to a bottom of the plastic microplate to heat the microplate through this block, and (3) a method in which high-temperature air is caused to flow between a heating plate and the plastic microplate to heat the microplate.
However, since the heat conductivity of a polymer is generally as low as about 0.5 W/mK, and so its thermal resistance is high, it has been difficult to cause wells in the plastic microplate to reach the target temperature in a short period of time even if these methods have been used.
Accordingly, even in the plastic microplates, there are demands for using thermoplastic resins excellent in heat resistance, chemical resistance, etc., such as PPS resins in place of the general-purpose polymers having poor heat resistance, and imparting high thermal conductibility to such resins.
As a method for improving the thermal conductibility of the PPS resin, it is considered to compound an inorganic filler having a high heat conductivity into such a resin. A typical inorganic filler having a high heat conductivity is metallic powder. When the metallic powder having a high heat conductivity is compounded into the PPS resin, however, the electrical insulating properties of the resin are impaired, and so such a composition cannot be applied to application fields of which high electrical insulating properties are required. Many of other inorganic fillers than the metallic powder are not always high in heat conductivity.
By the way, alumina among inorganic fillers is known as a material which has a heat conductivity at least 4 times as high as that of crystalline silica and is also chemically stable. However, general-purpose alumina has involved such problems that its filling ability to resins is poor, and the particles thereof are angular and hence have high wearability, and so such alumina is insufficient in utility as a filler. When the general-purpose alumina is filled in a great amount into a PPS resin to enhance the heat conductivity of the resin, there has been a problem that the melt-flow properties of the resin is deteriorated, and so injection molding becomes infeasible.
In order to enhance the heat conductivity of the PPS resin, there have heretofore been proposed (1) a resin composition for sealing electronic parts, comprising a PPS resin, and alumina, boron nitride and fibrous reinforcing material compounded into the resin (Japanese Patent Application Laid-Open No. 198265/1992) and (2) a resin composition comprising a PPS resin and at least one compounding ingredient selected from the group consisting of metal oxides, metal nitrides and boron nitride (Japanese Patent Application Laid-Open No. 198266/1992).
However, Examples of (1) Japanese Patent Application Laid-Open No. 198265/1992 and (2) Japanese Patent Application Laid-Open No. 198266/1992 only disclose resin compositions comprising alumina in a proportion of 10 to 40 wt. %, and resin compositions comprising alumina in a proportion of 30 to 50 wt. %, respectively. Therefore, no resin composition comprising alumina filled in a high proportion exceeding 50 wt. % is specifically disclosed.
On the other hand, according to the results of an investigation by the present inventors, it has been found that in order to provide a resin composition having excellent thermal conductibility as demonstrated by a heat conductivity of at least 1.5 W/m.multidot.K, preferably at least 2.0 W/m.multidot.K, more preferably at least 2.5 W/m.multidot.K using a PPS resin, it is necessary to compound alumina into the PPS resin in a proportion of at least 55 wt. %, preferably at least 70 wt. %, more preferably at least 75 wt. % based on the total weight of the resin composition. However, mere filling of the alumina in such a high proportion into the PPS resin involves a problem that no resin composition having satisfactory physical properties can be prepared or that the melt-flow properties of the resulting resin composition are deteriorated, and so such a composition is difficult to apply to melt processing techniques such as injection molding.
In order to apply a PPS resin composition to a use of microplates, it is necessary to limit the content of metallic ions, particularly, a Mg ion in the PPS resin to an extremely low level. The reason for it is that the Mg ion inhibits the activity of a Tag polymerase used in the PCR method.